The present invention relates to apparatus for generating an electric plasma under subatmospheric pressure; more particularly, to such apparatus including a magnet for producing a magnetic field in the plasma, and a sputtering cathode; and most particularly to such apparatus wherein the magnetic field is unbalanced, the sputtering cathode has cylindrical symmetry, and sputtering proceeds from a cylindrical target surface. Apparatus and methods in accordance with the invention are useful for magnetron sputtering deposition of materials on substrates and for plasma cleaning or etching of substrates.
Cylindrical magnetron sputtering is a useful method for coating target materials onto three-dimensional complex shapes, such as the shapes of cutting tools, forming tools, biomedical devices, optical fibers, and the like. Some relevant prior art cylindrical sputtering devices using balanced magnetrons are disclosed in U.S. Pat. Nos. 3,884,793; 3,995,187; 4,030,986; 4,031,424; 4,041,353; 4,111,782; 4,116,793; 4,116,794; 4,132,612; and 4,132,613, the relevant disclosures of which are incorporated herein by reference.
In planar magnetron sputtering, a known useful method for increasing the plasma density near the substrate(s) is to use an unbalanced magnetron, wherein part of the magnetic field which normally confines electrons to the vicinity of the target in balanced magnetrons is allowed to extend away from the target surface toward the substrate on which the film is being deposited. Thus, some of the field lines that emanate from the target surface do not close back through the target surface. This allows electrons, which have high mobility in directions parallel to the magnetic field lines, to be accelerated away from the target to where they can desirably ionize the background working gas in the vicinity of the substrate. Window and Savvides first disclosed unbalanced magnetrons (Journal of Vacuum Science and Technology A4, 196 (1986) and Journal of Vacuum Science and Technology A4, 453 (1986)).
Prior art unbalanced magnetron sputtering devices deal exclusively with planar, often circular, target surfaces. The xe2x80x9copenxe2x80x9d or unbalanced magnetic field lines can project from either the radially outer portions or the radially inner portions of the target surface. The targets can be circular, rectangular, or any other planar shape. The magnetic polarity is also unimportant in that either the north or south magnetic pole can be used to create the open or unbalanced field lines in all cases. In one common design, several unbalanced magnetron cathodes are used in concert to form a plasma trap surrounding the substrate (s). Such devices are taught in U.S. Pat. Nos. 5,196,105 and 5,556,519. This multiple cathode arrangement is particularly beneficial in large coaters used to deposit hard and corrosion resistant materials. Most often the higher plasma densities produced by unbalanced magnetron sputtering are utilized by applying a negative electrical bias to the substrate, which bias accelerates ions toward the growing film with resulting improvements in density, composition and microstructure. Therefore, the highest possible ionization density is generally desirable.
Prior art cylindrical magnetrons use any of a variety of means to create traps for the secondary electrons produced by ion bombardment of the target, which electrons are responsible for maintaining the plasma. Some traps are formed by axial magnetic fields working together with electrostatic wings, and others are created by the magnetic field together with the surface of the electrode. Examples of such traps are described in the referenced patents.
As in planar magnetron sputtering, any of the applications for which cylindrical magnetron sputtering is particularly well-suited, such as coatings for wear and corrosion resistance and dense dielectric coatings, can benefit from a high level of ionization in the vicinity of the part(s) being coated. This permits the effective use of substrate bias or creates a level of self-bias on non-conducting substrates that improves the physical properties of the deposited film through controlled levels of ion bombardment. However, prior art cylindrical magnetrons purposely confine the plasma to the vicinity of the target surface to enhance rates of removal of material from the target, and consequently the plasma density near the depositional substrate is reduced.
The present invention relates to an improvement in plasma generating equipment for creating high plasma densities in the vicinity of a substrate. An important application is in magnetron sputter deposition onto a substrate when aided by ion bombardment of the substrate. However, we have found also that cylindrical plasma generating devices described in accordance with the present invention may be useful for other applications, such as for cleaning or etching a substrate and for creating high intensity light emission.
In conventional planar unbalanced magnetron coating devices, the unbalanced flux lines emanating from the target surface either diverge, causing the plasma to grow weaker, or are captured by opposite-polarity magnets across the working space, forming a magnet xe2x80x9clinkage.xe2x80x9d In contrast, the unbalanced flux lines emanating from cylindrical magnetrons in accordance with the present invention cannot be captured across the working space, because the magnets are also cylindrical, and instead converge toward the axis of the cylinder to provide a high flux density, and therefore a high plasma density, in the vicinity of a substrate disposed in this region. In prior art sputter deposition, substrates being coated may be exposed to non-uniform regions of plasma and coating material as they move through a sputtering chamber. In sputter deposition in accordance with the present invention, the plasma profile and the coating material profile are both cylindrically symmetrical, resulting in a consistent and predictable coating on substrates.